In a color display tube the earth's magnetic field deflects electron paths, which deflections without any correcting measures may be so large that the electrons impinge upon the wrong phosphor, i.e. a mislanding, and produce a discoloration of the displayed picture. Particularly the component of the earth's magnetic field in the direction of the axis of the display tube (commonly referred to as the axial field) plays an important role in this respect, which may become manifest as a lack of color or even as color impurity in the corners of the display screen.
A known measure of reducing mislandings due to the earth's magnetic field is the use of an internal magnetic shield. The shape of such a shield, which is usually made of iron, roughly follows the contours of the envelope of the display tube. This means that the funnel-shaped shield has two long trapezoidal walls which are parallel to the long axis (the x axis) of the display screen and two short trapezoidal walls which are parallel to the short axis (the y axis) of the display screen.
The short sides of the shield are often provided with a V-shaped recess at the gun side so as to reduce mislandings in the corners due to the axial field. When relatively small tubes and a relatively large pitch of the pixels of the phosphor line pattern on the display screen are used, an acceptable result is achieved in this way. When larger display tubes and/or a smaller pitch of the phosphor pixels are used, this type of solution does not, however, guarantee a sufficient color purity. The invention is based on the recognition that mislandings due to the vertical component of the earth's magnetic field is increased because the short sides are provided with V-shaped recesses and that this effect is more serious as the size of the V-shaped recesses increases (which size depends on the strength of the earths magnetic axial field to be compensated).